Multilayers composed of a plurality of layers are used to achieve optimal reflectivity on optical elements for the EUV and soft x-ray wavelength region. Such multilayers are composed from periodic repetitions, a period consisting of two layers in the elementary case. As a rule, one layer material should have the highest possible index of refraction and slight absorption, while the other layer material should have the lowest possible index of refraction. The layer with the high index of refraction and slight absorption is also known as a spacer, the layer with low index of refraction is also called an absorber. The period thickness and the thicknesses of the individual layers are chosen such, in dependence on the operating wavelength, the mean angle of incidence, and the angle bandwidth of the incident radiation, that the integrated reflectivity over the illuminated surface is maximized. By a cover layer system is meant the portion of a multilayer coating or an optical element that is no longer periodic and forms a closure at the free interface. In the elementary case, this is merely the last individual layer.
Reflective optical elements are used, for example, in EUV lithography instruments for the production of semiconductor components. In use, they are exposed to both an irradiation of up to 20 mW/mm2 EUV intensity or more, and to a residual gas fraction of water, oxygen and hydrocarbons, as well as other residual gas components. Residual gas components adsorbed onto irradiated surfaces are split up into reactive cleavage products by photoinduced electrons due to bombardment of the surface with EUV photons. This generally leads to a degradation or contamination, e.g., by oxidation, carbon deposits, interdiffusion, material ablation, etc., of the multilayer surface. These effects lead to imaging errors and transmission losses. In the worst case, the desired imaging is totally impossible. Thus, regeneration cycles must be provided during operation of the EUV lithography machine, which not only significantly increase the operating costs, but also in the extreme case lead to an irreversible damaging and, thus, may entail a replacement of the affected reflective optical elements.
Thus far, one has attempted to counter a negative alteration of the surface by providing a cover layer system on the surface of the reflective optical element, supposed to protect the surfaces. The basic layout of traditional reflective optical elements is sketched in FIGS. 1a to c. These show three different multilayer systems 2. In FIG. 1a, a multilayer system 2 is shown in which the layer thicknesses are constant both along the thickness of the multilayer system 2 and also across the surface. FIG. 1b shows a multilayer system 2 in which the thickness relations of a period are constant along the entire depth, but there is a nonconstant distribution of thicknesses in the surface, and so the multilayer system 2 has a lateral gradient. In FIG. 1c, the multilayer system 2 does not have a lateral gradient, but the distribution of layer thicknesses varies across the depth of the multilayer system (so-called depth-graded multilayer). All multilayers are deposited on a substrate 3. Beneath the multilayer system 2, a portion of the substrate 3 is configured as an optically shaping region 5, also known as a shaper. The shaper 5 is needed primarily to give the optical element 1 a shape which leads to the desired optical properties. The multilayer system 2 borders on a cover layer system 6, which in FIGS. 1a to c consists of two segments 7, 8, one segment 7 generally serving to adapt the phase to the multilayer system or to the interdiffusion protection and the second segment 8 generally serving for the actual contamination protection. The boundary surface 4 of the cover layer system 6 next to the vacuum is known as the free boundary surface 4.
Thus far in the prior art it has been attempted to positively influence the contamination and degradation of the reflective optical element by the choice of specific cover layer materials. Thus, for example, U.S. Pat. No. 6,228,512 proposes having a protective layer of SiO2, Zr2O or ZnO on a MoRu/Be multilayer, which does not react with water. In particular, ZnO is recommended, for when zinc is applied there is formed a ZnO layer only 0.5 to 0.6 nm thick, which sufficiently protects the multilayer against oxidation, without significantly impairing the reflectivity—because of its slight thickness.
U.S. Pat. No. 5,958,605 proposes a special protection layer system for EUV multilayers in which a lower layer of silicon or beryllium is proposed, placed directly on the multilayer, and at least one top layer is applied onto the lower layer, and this top layer has a material which is resistant to oxidation and corrosion and also protects the underlying layers against oxidation.
While the protective layers of U.S. Pat. No. 5,958,605 and U.S. Pat. No. 6,228,512 provide a protection against degradation by the influence of oxygen, there still occurs a contamination from carbon-containing substances. These lead to uncontrolled losses of reflectivity and changes in the wave front.